1. Field of the Invention
This invention relates to an operational amplifier circuit having a low power-supply voltage for handling differential signals, and more specifically to an operational amplifier circuit capable of improving frequency characteristics and decreasing a common-mode signal, and a sample/hold circuit and a filter circuit using the operational amplifier circuit.
2. Description of the Related Art
The progress of integrated circuits is remarkable and the fine fabrication process of integrated circuits progresses year by year. Although the finer fabrication process improves the performance of transistors per se, a breakdown voltage of a transistor is getting lower. Thus, the applicable power-supply voltage is getting lower. When the power-supply voltage is low, the amplitude of voltage signals in integrated circuits decreases, which makes it difficult to achieve a desired signal-to-noise ratio (S/N ratio). In order to solve this problem, a single-ended signal, which has been used, is changed to a differential signal to thereby double the signal amplitude as compared with the single-ended signal amplitude.
In a balanced operational amplifier circuit, which handles differential input/differential output, however, it is necessary to suppress a common-mode signal. If the elimination of the common-mode signal of the operational amplifier circuit is insufficient, the voltage range of the output decreases, which causes a distorted differential signal. Especially at a low power-supply voltage, since the available voltage range is limited, the suppression of a common-mode signal is essential.
Conventionally, in order to eliminate a common-mode signal, a common-mode feedback circuit was used (see, e.g., Japanese Patent Disclosure KOKAI P2000-148262). The design of this common-mode feedback circuit is complicated as compared with a differential-input circuit of a single-ended output, and defects such as oscillation tend to occur.
A balanced-type amplifier circuit was proposed in which amplifier circuits having a plurality of input terminals and output terminals are combined to decrease common-mode signal components (see, e.g., U.S. application Ser. No. 10/281,103 filed on Oct. 28, 2002).
Since the aforementioned conventional amplifier circuit can be realized by a simple structure in which inverted amplifier circuits are combined, a low power-supply voltage can be realized. However, in cases where such an amplifier circuit is constituted by two high gain stages in order to obtain enough gain, since the polarity must be reversed, it is necessary to use low gain stages whose gain is approximately 1. Accordingly, the number of internal nodes from the input to the output becomes 2 (two). As the number of internal nodes increases, the frequency characteristic deteriorates due to the parasitic capacitance in each node.
It is an object of the present invention to provide an operational amplifier circuit of a balanced structure using two gain stages appropriate for a low power-supply voltage, and in particular an operational amplifier circuit with sufficient common-mode rejection ratio and with a frequency characteristic improved by limiting the number of internal nodes in the differential signal path to 1(one), and a sample/hold circuit and a filter circuit using the operational amplifier circuit.